mise's ${VAR:-fallback} deviates from POSIX shell :- semantics

[gerhard]

Follow-up to:

• Replace asdf, just, dagger & direnv with mise + fnox thechangelog/changelog.com#557 (comment)

Summary

With env_shell_expand = true, mise's ${VAR:-fallback} deviates from POSIX shell :- semantics in two ways:

1. Empty strings aren't coalesced. ${VAR:-fallback} keeps an empty value (VAR="" → "") instead of falling back. POSIX :- covers both unset and empty; mise's behaves like - (unset only).
2. The fallback branch isn't expanded. ${UNSET:-$OTHER} yields the literal string $OTHER rather than the value of $OTHER.

Reproduced on 2026.6.6 and 2026.6.9 (linux-x64).

Repro

mise.toml:

[settings]
env_shell_expand = true

[env]
BUG1 = "${BUG1:-fallback}"
BUG2 = "${UNSET_VAR:-$OTHER}"

$ mise version
2026.6.9 linux-x64 (2026-06-14)

# BUG1 — empty string should coalesce to the default
$ BUG1='' mise env | grep BUG1
set -gx BUG1 ''                       # ← expected: fallback
$ BUG1='' sh -c 'echo ${BUG1:-fallback}'
fallback                              # ← POSIX

# BUG2 — fallback branch should be expanded
$ OTHER=hello mise env | grep BUG2
set -gx BUG2 '$OTHER'                 # ← expected: hello
$ OTHER=hello sh -c 'echo ${UNSET_VAR:-$OTHER}'
hello                                 # ← POSIX

(VAR=value mise env correctly returns value, and a truly-unset VAR correctly returns the fallback — only the empty-string and fallback-expansion cases differ.)

Why it matters

CI systems commonly forward variables unconditionally, so an unset variable arrives as an empty string, not as truly unset. For example, GitHub Actions:

env:
  IMAGE_OWNER: ${{ vars.IMAGE_OWNER }}   # becomes "" when the repo var is unset

With the empty-coalesce gap, IMAGE_OWNER = "${IMAGE_OWNER:-thechangelog}" resolves to "" in that case rather than the intended default — so the shell-expansion form can't replace the equivalent Tera coalesce:

IMAGE_OWNER = "{% set v = get_env(name='IMAGE_OWNER', default='') %}{% if v %}{{ v }}{% else %}thechangelog{% endif %}"

Expected

${VAR:-default} / ${VAR:-$OTHER} should match POSIX :-: use the default when the variable is unset or empty, and expand variable references inside the fallback.

(If :- is intentionally unset-only, supporting ${VAR:=default}/:?/etc. or documenting the divergence would still help; and the ${VAR-default} vs ${VAR:-default} distinction would be a natural way to opt into the unset-only behavior.)

[jdx]

Thanks for the detailed repro. I opened #10445 to make env shell expansion match the documented/POSIX-style behavior for these cases: ${VAR:-default} now falls back when the value is empty, ${VAR-default} keeps empty strings as set, and fallback branches like ${UNSET:-$OTHER} are shell-expanded.

Validated with mise run test:e2e e2e/env/test_env_shell_expand and mise run format.

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